No. 6 was a very old “spent” liquor. The mean hydrogen ion concentration before goods was 0·588 volt, i.e. the concentration was 10^-5·32 normal equivalent to 0·00000479 grm. per litre of hydrogen ions. Therefore the value of p+H was 5·32. A solution of hydrochloric acid of the same strength by titration consumed 0·7 c.c. N/1 alkali per 100 c.c. Measured by the electrometric apparatus, it showed 0·410 volt, corresponding to p+H = 2·1, or a hydrion concentration of ·0079N. In other words, the HCl solution has an acidity or strength 1600 times that of the puer liquor.

The mean hydrogen ion concentration of the liquors after goods was 0·000000076 grm. per litre, corresponding to 0·715 volt and p+H = 7·12, i.e. the liquor was alkaline to a slight extent. For comparison saturated lime-water gave a reading of 1·01 volt, corresponding to p+H = 12·5.

The hydrogen ion concentration is of the greatest importance for the proper action of the enzymes in the bate;70 we shall, however, treat of this in Chapter VII.

Conductivity of Puer Liquors.—It was thought of interest to examine the electrical conductivity of puer liquors in actual use, in the hope that the numbers obtained might give some useful indications. It was found that the conductivity increased, as might be expected from the lime going into solution, but the difficulties of the method render it of less use than ordinary chemical analysis. The results of a typical liquor are given here as a record—

Conductivity (K) of liquor before goods

0·00316 1/ohm × cm.

Conductivity (K) of liquor after goods

0·00423 1/ohm × cm.

The difficulty of expressing the complex reactions of puering numerically is, we have seen, very great, for, as Minot71 says, “with human minds constituted as they actually are, we cannot anticipate that there will ever be a mathematical expression for any organ or even a simple cell, although formulæ will continue to be useful for dealing now and then with isolated details. Nevertheless, the value of graphic methods to every student of science has been immense.”

It has long been my endeavour to express quantitatively the degree to which a skin has fallen. My friend Dr. Sand has suggested that this may be done by subjecting a piece of the skin successively to increasing and then decreasing pressures, and measuring the thickness under each load. Experiments carried out with the apparatus described below show that a limed skin treated in this way is first compressed, and then on releasing the pressure recovers more or less of its former thickness, according to the amount of plumping it has received, i.e. it shows a certain amount of resilience. A well-puered sheep-skin, on the other hand, shows no resilience at all, i.e. on releasing the pressure the whole of the compression persists. In the case of an ox-hide subjected to a bate of hen-dung, a slight recovery takes place on releasing the pressure. This accords with the fact that it will never be possible to puer a thick ox-hide so effectively as a thin sheep-skin. A piece of india-rubber, on the other hand, is completely resilient, i.e. it wholly recovers its thickness on releasing the pressure. The relative thickness of the same skin in the limed and puered conditions under varying loads is also of interest. The process of puering may, as a rule, be taken to reduce a limed skin to between two-thirds and one-half of its thickness in the swollen condition. If both limed and puered skin be then subjected to the same load, the puered skin will at first be compressed very much more than the limed one. This is probably due to the expulsion from it of water, held simply by capillary attraction. On further increasing the load, however, the compression decreases greatly in the case of the puered skin; with both limed and puered skin increase of compression ultimately becomes practically proportional to increase of pressure, and is slightly greater with the former than with the latter.

The table gives representative results obtained on the same sheep-skin (roan) in the limed and in the puered condition. These results are expressed graphically in Fig. 12.

Δ = difference in thickness of the skin—i.e. compression under the same load.

Fig. 11 shows the apparatus72 that was employed to obtain these results. It consists essentially of a commercial form of micrometer for measuring the thickness of leather. To one of its jaws a pan for weights is attached, by means of the frame f f, in such a manner as to secure a perfectly straight pull. The weights of the frame and pan are counterbalanced in the manner shown by a counterpoise b. The delicacy of measurement may be increased by inserting larger jaws in the form of suitably fashioned disks, but even when this is done the results are to a certain extent vitiated by the rather considerable friction of the micrometer.

An apparatus free from this fault is shown in Fig. 13. It consists essentially of a counterbalanced lever A, to which the upper jaw J is rigidly attached. By means of a sliding weight W, any desired load, from zero upwards, may be put on this jaw. The lever carries a very delicate spirit level, which allows it to be set accurately horizontal in every experiment. The lower jaw is movable vertically between parallel guides, and its position is controlled by the screw-wheel S which bears a divided circle on its circumference. The position of this wheel, and therefore of the lower jaw, may be accurately read on the vernier v. In every experiment it is adjusted so as to make the upper lever accurately horizontal.